Different Types of
Corrosion- Recognition, Mechanisms & Prevention

High-temperature Hydrogen Attack
(Decarburization)

Recognition of High Temperature Hydrogen Attack (HTHA)

What is high-temperature hydrogen
attack?

High-temperature Hydrogen Attack (HTHA) refers to the loss of
strength and
ductility of steel by high-temperature reaction of absorbed
hydrogen with carbides in the steel, resulting in decarburization and
internal fissuring.

High-temperature hydrogen attack is also referred to as
hot hydrogen attack or decarburization. It occurs in carbon
and low-alloyed steels exposed for an extended period to hydrogen under high
pressure and at high temperature.

This micrograph shows intergranular cracking
caused by high temperature hydrogen attack in a C-0.5Mo steel.

Mechanisms of High
Temperature Hydrogen Attack

What causes High-temperature hydrogen attack?

Atomic hydrogen (formed in
corrosion processes or by
dissociation of molecular hydrogen in a gas stream at the steel surface)
diffuses into steel. At grain boundaries, crystal imperfections, inclusions,
discontinuities and other defects, the atomic hydrogen reacts with the
dissolved carbon or with the metal carbides, forming methane:

Because of the pressure build-up of the methane in the
steel, this results in the formation of intergranular cracks (refer to the
micrograph above), fissures and
blisters, often extending to the surface of the steel. This form of hydrogen
damage sometimes resembles the low-temperature
hydrogen blistering.
Moreover, the decarburization process leads to the loss of carbon in the
steel and hence a reduction in tensile strength and an increase in ductility
and creep rate. Interestingly, the reverse process (Eq.1 above),
carburization, can also occur in hydrogen-hydrocarbon mixtures such as that
encountered in petroleum-refining operations.

Nelson curves are commonly used to select the
various grades of steels and the safe operating limits of temperature and
hydrogen partial pressure.

Prevention of High
Temperature Hydrogen Attack

How to prevent
High-temperature hydrogen attack? High-temperature hydrogen
attack,
decarburization and fissuring can be prevented through:

avoid high carbon steels

use higher alloyed steels

following the safe operating limits
defined in the Nelson curves.

use a safety margin of 30°C when using
Nelson curves

For more details on High
Temperature Hydrogen Attack

Where can I learn more about
high temperature hydrogen attack? More details on high-temperature
hydrogen attack are included in the following
corrosion courses which you can take as
in-house training courses,
course-on-demand, online
courses or distance
learning courses: